An ultrasonic wave is commonly defined as a wave of 16,000 Hz or more. It allows the internal state of an object to be checked in a nondestructive manner, without the interior being harmed, and is utilized in various fields for examination of a defect, diagnosis of a disease and others. One of the applications of an ultrasonic wave is found in an ultrasonic diagnostic apparatus, wherein the interior of a test object is scanned by an ultrasonic wave, and the internal state of the test object is formed into an image, based on the reception signal generated from reflected ultrasonic wave (echo) from inside the test object. This ultrasonic diagnostic apparatus uses an ultrasonic probe for transmitting and receiving the ultrasonic wave to and from the test object. This ultrasonic probe is formed, for example, of a two-dimensional array of ultrasonic transmitting/receiving elements. These ultrasonic transmitting/receiving elements are provided with piezoelectric elements that cause a mechanical vibration and generate an ultrasonic wave, based on the transmission signal, and generate a reception signal by receiving the reflected wave of the ultrasonic wave produced by the mismatching of acoustic impedance inside the test object.
In recent years, efforts have been made to research and develop a harmonic imaging technique which, instead of using the frequency (fundamental frequency) component of the ultrasonic wave transmitted from the ultrasonic probe into the test object, employs the harmonic frequency component thereof to form an image of the internal state of the test object. This harmonic imaging technique has various forms of advantages such as the contrast resolution enhanced by the improvement of the S/N ratio (signal-to-noise ratio) by the side lobe level that is smaller than the level of the fundamental frequency component, the lateral resolution improved by the beam width reduced by a higher frequency, the multiple reflection reduced by a smaller sound level and smaller fluctuation in the sound level at a short distance, and sensitivity in the depths allowed in a high level as compared to the case where a high frequency wave is used as the fundamental wave, because attenuation beyond the focus is on the level of that of the fundamental wave.
The ultrasonic probe for the harmonic imaging requires a wide range of frequency bands extending from the frequency of a fundamental wave to the frequency of a harmonic wave. The frequency-domain on the low frequency side is used for transmission to transmit the fundamental wave, and the frequency-domain on the high frequency side is used for reception to receive the harmonic wave. This ultrasonic probe for harmonic imaging is exemplified by the apparatus disclosed in the Patent Literature 1.
The ultrasonic probe disclosed in the Patent Literature 1 is applied to a test object, and receives the ultrasonic wave returned by reflection from inside the test object after having been sent into the test object. The ultrasonic probe includes a first piezoelectric layer that includes a plurality of arranged first piezoelectric elements having a predetermined first acoustic impedance. This first piezoelectric layer takes charge of transmission of the fundamental wave consisting of ultrasonic waves having a predetermined center frequency, toward the interior of a test object, and reception of the fundamental wave out of the ultrasonic wave having been returned by reflection from the inside of the test object. Further the ultrasonic probe includes a second piezoelectric layer that includes a plurality of arranged piezoelectric elements having a predetermined second acoustic impedance smaller than the first acoustic impedance. The aforementioned second piezoelectric layer is overlapped on the entire surface of the first piezoelectric layer on the side, the ultrasonic probe of which is applied to the test object, and is in charge of reception of the harmonic wave out of the ultrasonic wave having been returned by reflection from the inside of the test object. The ultrasonic probe disclosed in the Patent Literature 1 uses the aforementioned structure to transmit and receive ultrasonic waves in a wide frequency band.
Patent Literature 1: Japanese Unexamined Patent Application Publication No. Hei 11-276478